1. Technical Field
The present invention relates to a method and an apparatus for producing bottle-type synthetic resin containers, and, in particular, provides a production technology that allows crystallization step for the neck portion of such container to be performed in a short time.
2. Prior Art
In bottle-type synthetic resin containers, in order to improve the heat resistance and durability at the neck portion of the container, it is known to heat and crystallize the neck portion of the corresponding perform. As opposed to the body portion of the container in which molecules are oriented by biaxial stretching blow molding to preserve transparency, the neck portion subjected to crystallization treatment has a crystal structure in which sphaerites are grown to undergo devitrification into white or milky-white color. A method for producing such bottle-type container is more fully disclosed, e.g., in U.S. Pat. No. 4,375,442.
Crystallization of synthetic resin occurs when the resin is heated under a temperature condition not lower than its glass transition temperature. Generally, in the case of polyethylene terephthalate (PET) resin, crystallization is carried out by heating the neck portion of the preform to a temperature not lower than approximately 120° C. for a predetermined duration and, it is possible to increase the crystallizing speed when heating is performed within a temperature range of approximately 170° C. to 190° C.
However, in the conventional crystallizing process, because the temperature of the neck portion of the preform is elevated immediately after starting the heating to the above-mentioned crystallizing temperature range gradually, a relatively long time is required until crystallization starts, thereby imposing a limitation to improve the productivity.